Method for fluorination of actinide fluorides and oxyfluorides thereof using O2 F2

ABSTRACT

Method for fluorination of actinides and fluorides and oxyfluorides thereof using O 2  F 2  which generates actinide hexafluorides, and for removal of actinides and compounds thereof from surfaces upon which they appear as unwanted deposits. The fluorinating agent, O 2  F 2 , has been observed to readily perform the above-described tasks at sufficiently low temperatures that there is virtually no damage to the containment vessels. Moreover, the resulting actinide hexafluorides are thereby not destroyed by high temperature reactions with the walls of the reaction vessel. Dioxygen difluoride is easily prepared, stored and transferred to the desired place of reaction.

This invention is the result of a contract with the Department of Energy(Contract No. W-7405-ENG-36).

BACKGROUND OF THE INVENTION

The present invention relates generally to methods of fluorination andmore particularly to the use of O₂ F₂ for the preparation of actinidehexafluorides, and for the extraction of deposited actinides andfluorides and oxyfluorides thereof from reaction vessels.

Fluorinating agents useful for removing deposited actinide metal andcompounds thereof from surfaces, and for preparing volatile actinidehexafluorides from their very stable solid tetrafluorides, oxyfluoridesand pentafluorides are known. However, the difficulty with all of theknown materials which are capable of performing these tasks is thatlittle reaction takes place at or below room temperature. The requisitehigh temperatures (typically in excess of 300 C.) and harsh oxidizingenvironments result in the deterioration of any containment vessels andtransfer equipment as well as in the destruction of the articles to becleaned and the intended end products, the actinide hexafluoridesthemselves. Moreover, the reaction of the fluorinating agents with everywarm surface in their pathway has made it very difficult to introducethe fluorinating agent into regions of interest. Atomic fluorine, apreferred fluorinating agent for the above-described tasks, is usuallygenerated by microwave radiation, but the overall fluorination processis very inefficient.

Dioxygen difluoride was first prepared and isolated in 1933. A carefulinvestigation of the properties of this material was performed by A. G.Streng in and presented his article entitled "The Oxygen Fluorides"published in Chem. Rev. 63, 607 (1963), the disclosure therein herebybeing incorporated by reference herein. Of particular interest in thisarticle is the mention by the author of the explosive reaction of O₂ F₂when placed in contact with a sheet of platinum covered with platinumfluoride at 160 K. Even at this low temperature, it would appear that O₂F₂ is unstable in the presence of metallic surfaces. The use of O₂ F₂for removing actinides and/or actinide containing compounds from metalsurfaces would then be impossible especially if the O₂ F₂ had to beintroduced into the region of interest through metallic plumbing.

SUMMARY OF THE INVENTION

One object of the subject invention is to provide a method for preparingactinide hexafluorides from their respective tetrafluorides,pentafluorides and/or oxyfluorides.

Another object of our invention is to provide a method for removingactinides and/or actinide containing compounds from metallic surfaces.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

To achieve the foregoing and other objects, and in accordance with thepurpose of the present invention, as embodied and broadly describedherein, the method of this invention includes contacting surfaces fromwhich actinides or actinide-bearing compounds are to be removed in theform of their respective hexafluorides or contacting actinide fluoridesand/or oxyfluorides from which the respective hexafluorides are to beprepared, with O₂ F₂ for a time period sufficient to obtain substantialreaction between the surfaces from which actinides or actinide-bearingcompounds are to be removed, or the actinide fluorides and/oroxyfluorides, removing the products of the reaction from the reactionregion, and separating and collecting the actinide hexafluoridesproduced. Preferably, the fluorination step is performed at temperaturesbelow approximately 25° C. Preferably also, the fluorination step isperformed in the temperature range between about -80° and 25° C. It isalso preferred that the O₂ F₂ be flowed in the fluorination step.

In a further aspect of the present invention, in accordance with itsobjects and purposes, the method hereof includes contacting the surfacesfrom which actinides or actinide-bearing compounds are to be removed, inthe form of their respective hexafluorides, or contacting actinidefluorides and/or oxyfluorides from which the respective hexafluoridesare to be prepared, with a fluorinating agent which consists essentiallyof O₂ F₂ for a time period sufficient to obtain substantial reactionbetween the surfaces from which actinides or actinide-bearing compoundsare to be removed, or the actinide fluorides and/or oxyfluorides,removing the products of the reaction from the reaction region, andseparating and collecting the actinide hexafluorides produced.Preferably, the fluorination step is performed at temperatures belowapproximately 20 C. Preferably also, the fluorination step is performedin the temperature range between about -80° and 25° C. It is alsopreferred that the O₂ F₂ be flowed in the fluorination step.

The subject invention, then, is a method for generating actinidehexafluorides from non-volatile actinide fluorides and/or oxyfluorides,and for removing actinides and actinide-bearing compounds from surfaces,utilizing the exceptional fluorinating properties of O₂ F₂. Suchfluorinations can be achieved at low temperatures which simultaneouslyimproves the efficiency of the fluorination, avoids the destruction ofthe containment vessels and avoids the destruction of the actinidehexafluorides formed as a result of the fluorination process and whichare the desired products to be recovered.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present preferredembodiments of the invention, which are illustrated in the accompanyingexamples.

EXAMPLE 1

Reaction of O₂ F₂ with photochemically deposited PuF₄ : About 0.33 mmol(156 mg) of gaseous PuF₆ was transferred into a dry quartz U-tube andirradiated using a 450 W Hg ultraviolet lamp for 20 minutes.Approximately 110 mg of the PuF₆ was decomposed to form 98 mg (0.31mmol) of PuF₄ which deposited uniformly on the walls of the U-tube as awhite coating. The PuF₆ that had not photolyzed was removed.

About 3.7 mmol of O₂ F₂ was cryogenically transferred into the U-tubecontaining the PuF₄ through a 12 in. long stainless steel bellows usingliquid nitrogen, and the tube warmed to approximately 25° C. Afterseveral minutes, the condensable gas products were collected. The PuF₆formed was collected at -78° C. and pressure measurements showed thatabout 90% of the PuF₄ had reacted to form 99 mg of PuF₆. All visibleevidence of solids on the walls of the U-tube had disappeared.

EXAMPLE 2

Reaction of O₂ F₂ with PuO₂ F₂ : A small amount of water was placed in aquartz U-tube and allowed to stand for 30 minutes to saturate the glasssurfaces. The excess water was pumped off. Approximately 0.26 mmol ofgaseous PuF₆ was connected in the U-tube and allowed to react for threehours at room temperature. The PuF₆ was completely converted to PuO₂ F₂,most of which remained in the bottom of the U-tube where the PuF₆ wascondensed.

About 3.5 mmol of O₂ F₂ was condensed onto the PuO₂ F₂ through a 12 in.long stainless steel bellows using liquid nitrogen, and the twocompounds allowed to warm to approximately 25° C. After a short reactionperiod, 15.5 mg of PuF₆ (17% of starting material) was recovered. Thereaction with O₂ F₂ was repeated and additional PuF₆ (12%) wasrecovered.

EXAMPLE 3

Reaction of the O₂ F₂ with PuF₄ formed by thermal decomposition of PuF₆: About 300 mg of gaseous PuF₆ was decomposed in a prefluorinated Monelmetal U-tube by heating at 250° C. for 16 hours. Approximately 7 mg ofPuF₆ remained undecomposed and was removed. The U-tube was thenevacuated.

About 5.5 mmol of O₂ F₂ was condensed into the U-tube and then warmed toapproximately 25° C. About 22 mg of PuF₆ was recovered (7% yield).

In a second experiment, gaseous O₂ F₂ was flowed through the U-tube keptat 25 C. for 15 min. An additional 26 mg of PuF₆ (8%) was collected.

EXAMPLE 4

Reaction of PuF₄ with O₂ F₂ prepared in situ: 0.33 mmoles of gaseousPuF₆ was decomposed in a quartz bulb by uv irradiation. The whiteproduct, PuF₄, coated the walls of the bulb. After evacuation, F₂ and O₂were admitted, the reactor immersed in liquid N₂, and uv light admitted.Yellow, red O₂ F₂ formed on the walls. The excess F₂ and O₂ was pumpedaway and the reactor then allowed to warm slowly to room temperature. Ameasurement of the F₂ and O₂ pressure after reaction indicated 6.3 mmolof O₂ F₂ had formed.

The PuF₆ was collected at -78° C. and measured. 85% of the original PuF₆was recovered. Very little of the white decomposition product remainedin the reaction vessel.

Examples 1-4 demonstrate the feasibility of the utilization of O₂ F₂ fordecontamination of difficult-to-access surfaces from the nonvolatilefluorides and oxyfluorides of plutonium and other actinides. No otherknown reagent other than F-atoms is capable of PuF₆ generation at thelow temperatures used here. As mentioned hereinabove, however, F-atomgeneration for fluorination processes is inefficient.

The low yields observed in Examples 2 and 3 reflect the slower reactionrates which occur when O₂ F₂ reacts with less reactive solid compounds.That is, PuO₂ F₂ and highly crystalline PuF₄ obtained by thermaldecomposition of PuF₆, are much less reactive than the finely dividedPuF₄ resulting from the photochemical decomposition of PuF₆. In general,with O₂ F₂, higher reaction yields can be obtained by lowering thereaction temperature, thereby giving longer contact time with the O₂ F₂.In Examples 1-4, no attempt was made to optimize the PuF₆ yield.

Further, examples 1-4 show that the very reactive O₂ F₂ is capable ofoxidizing PuF₄ to PuF₆ at low temperatures. Total plutonium removal fromcontaminated metal surfaces has been repeatedly observed after treatmentwith O₂ F₂ gas. It has also been observed that O₂ F₂ rapidly convertsUF₄ to UF₆, demonstrating thereby applicability of our method to otheractinide elements.

A molecular species such as O₂ F₂ would be expected to survive longerand thereby be more useful in delivering F-atoms to a distant surface tobe fluorinated than F-atoms themselves. For example, O₂ F₂ was shown tosurvive passage through about 12 in. of stainless steel bellows atambient temperature.

In summary, the potency of O₂ F₂ as an oxidative fluorinating agentpermits reactions to be run at low temperatures and thus avoids many ofthe problems of other fluoride volatilization processes contemplated forplutonium recovery. Moreover, despite its short lifetime at ambienttemperature, O₂ F₂ can be delivered effectively to relatively remotelocations and thus has important applications for plutoniumdecontamination. The experiments set forth hereinabove demonstrate thatthe room temperature or below use of O₂ F₂ will be highly beneficial forthe preparation of pure actinide hexafluorides from their respectivetetrafluorides without traces of HF being present as occurs using otherfluorinating agents; and decontamination of equipment previously exposedto actinides; e.g., walls, feed lines, etc.

The foregoing description of several preferred embodiments of theinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

What we claim is:
 1. A method for the preparation of PuF₆ from PuF₄,which comprises the steps of:a. contacting the PuF₄ starting materialwith O₂ F₂, thereby generating PuF₆ ; and b. collecting said generatedPuF₆.
 2. The method as described in claim 1, wherein said O₂ F₂contacting step is conducted at below about 25 C., whereby substantialcorrosion of the reactant containing vessel is eliminated.
 3. The methodas described in claim 2, wherein said O₂ F₂ contacting step is conductedbetween about -80 and 25 C., whereby substantial corrosion of thereactant containing vessel is eliminated.
 4. A method for removal ofplutonium and plutonium-containing compounds from plutonium-contaminatedarticles, which comprises the steps of:a. introducing O₂ F₂ into theregion of the articles of interest; b. permitting contact with said O₂F₂ for a time period such that the plutonium and plutonium-containingcompounds are substantially removed from the plutonium-contaminatedarticles as a result of a chemical reaction with said O₂ F₂, wherebyPuF₆ is produced; and c. removing said PuF₆ from the region of theplutonium-contaminated articles.
 5. A method for the preparation of PuF₆from PuF₄, which comprises the steps of:a. contacting the PuF₄ startingmaterial with a fluorinating agent, which consists essentially of O₂ F₂,thereby generating PuF₆ ; and b. collecting said generated PuF₆.
 6. Amethod for removal of plutonium and plutonium-containing compounds fromplutonium-contaminated articles, which comprises the steps of:a.introducing a fluorinating agent, which consists essentially of O₂ F₂into the region of the articles of interest; b. permitting contact withsaid O₂ F₂ for a time period such that the plutonium andplutonium-containing compounds are substantially removed from theplutonium-contaminated articles as a result of a chemical reaction withsaid O₂ F₂, whereby PuF₆ is produced; and c. removing said PuF₆ from theregion of the plutonium-contaminated articles.
 7. A method for thepreparation of PuF₆ from PuF₄, which comprises the steps of:a.contacting the PuF₄ starting material with flowing O₂ F₂, therebygenerating PuF₆ ; and b. collecting said generated PuF₆.
 8. A method forremoval of plutonium and fluorides and oxyfluorides thereof fromplutonium-contaminated articles, which comprises the steps of:a. flowingO₂ F₂ into the region of the articles of interest; b. permitting contactwith said flowing O₂ F₂ for a time period such that the plutonium andplutonium-containing compounds are substantially removed from theplutonium-contaminated articles as a result of a chemical reaction withsaid O₂ F₂, whereby PuF₆ is produced; and c. removing said PuF₆ from theregion of the plutonium-contaminated articles.
 9. A method for thepreparation of actinide hexafluorides from the tetrafluorides thereof,which comprises the steps of:a. contacting the actinide tetrafluorideswith O₂ F₂, thereby generating the hexafluorides of the actinidespresent; and b. collecting said generated actinide hexafluorides.
 10. Amethod for removal of actinides and actinide-containing compounds fromactinide-contaminated articles, which comprises the steps of:a.introducing O₂ F₂ into the region of the articles of interest; b.permitting contact with said O₂ F₂ for a time period such that theactinides and actinide-containing compounds fluorides and oxyfluoridesthereof are substantially removed from the actinide-contaminatedarticles as a result of a chemical reaction with said O₂ F₂, wherebyactinide hexafluorides are produced; and c. removing said actinidehexafluorides from the region of the actinide-contaminated articles.